Problem: In chemistry class, the girl to boy ratio is $6$ to $4$. If there are a total of $30$ students, how many girls are there?
Explanation: A ratio of $6$ girls to $4$ boys means that a set of $10$ students will have $6$ girls and $4$ boys. A class of $30$ students has $3$ sets of $10$ students. Because we know that there are $6$ girls in each set of $10$ students, the class must have $3$ groups of $6$ girls each. There is a total of $18$ girls in chemistry class.